1. Field of the Invention
This invention relates to solid state imaging elements useful in electronic cameras and other related applications. It provides a class of high sensitivity image sensing elements which are assembled into arrays for monochromatic or full-color image sensing devices. These image elements are made of a thin layer (or several layers) of organic semiconductor sandwiched between two conductive electrodes having different or similar work functions. The image signal can be probed by a circuit connected with the two electrodes. The spectral response of the image sensors can be modified and adjusted to desired spectral profiles through material selection, through device thickness adjustment and/or through optical filtering. Several approaches for achieving red, green, and blue color detection or multiple color detection in other desired spectral ranges are disclosed. These sensing elements can be integrated or hybridized with other electronic or optical devices.
2. State of the Art
Several types of image sensor devices have been developed based upon opto-electronic effects in solid state materials. Examples include charge-coupled devices (CCDs), active photosensor arrays made with CMOS technology, and large size image sensors made by combining a matrix of amorphous silicon photocells and a matrix of thin field effect transistors, TFTs, [R. A. Street, J. Wu, R. Weisfield, S. E. Nelson and P. Nylen, Spring Mecting of Materials Research Society, San Francisco, April 17-21 (1995); J. Yorkston et al., Mat. Res. Soc. Sym. Proc. 116, 258 (1992); R. A. Street, Bulletin of Materials Research Society 11(17), (1992); L. E. Antonuk and R. A. Street, U.S. Pat. No. 5,262,649 (1993); R. A. Street, U.S. Pat. No 5,164,809 (1992)]. CCDs are integrating devices.; the accumulated charges generated by incident light intensity are sequentially passed to the end of each row of pixels. This operation mechanism places rigorous demands on material quality and processing conditions. These requirements make CCD arrays costly (.about.$10.sup.3 -10.sup.4 for a CCD camera with dimensions of 0.75@-1@) and thus limit commercial CCDs to sub-inch dimensions.
Recently, research and development on active-pixel photosensors with CMOS technology on silicon wafers were re-activated following advances of this technology to submicron resolution [For a review of recent progress, see: Eric J. Lerner, Laser Focus World 32(12) 54, 1996]. The CMOS technology allows the photocells to be integrated with both the driver and the timing circuits so that a mono-chip image camera can be realized. However, even with the state-of-art CMOS technologies (&lt;0.3 .mu.m resolution), there is still limited space (typically much less than 50% of the pixel area) available for the photocells. Most of the pixel area is occupied by the necessary electronic components (field effect transistors etc) of the driving circuit. The same problem also limits the active TFT matrices designed to be used for high pixel density (&gt;100 dpi) image sensing applications. To improve the fill factor (the ratio of sensor area/pitch area) to close to 100%, high sensitivity, processable, thin film photosensor arrays (each sensor is often referred to as an image element) are desired such that each element of said array can be hybridized on top of the driver pixels made by CMOS technology or TFT technology.
Photodiodes made with organic semiconductors are promising candidates for such applications. Although there were early reports, in the 1980s, of fabricating diodes with organic molecules and conjugated polymers, relatively small photoresponse was observed [for a review of early work on organic photodiodes, see: G. A. Chamberlain, Solar Cells 8, 47 (1983)]. In the 1990s, there has been progress using conjugated polymers as the photosensing materials; see for example the following reports on the photoresponse in poly(phenylenevinylene), PPV, and its derivatives: S. Karg, W. Riess, V. Dyakonov, M. Schwoerer, Synth. Metals 54, 427 (1993); H. Antoniadis, B. R. Hsieh, M. A. Abkowitz, S. A. Jenekhe, M. Stolka, Synth. Metals 64, 265 (1994); G. Yu, C. Zhang, A. J. Heeger, Appl. Phys. Lett. 64, 1540 (1994); R. N. Marks, J. J. M. Halls, D. D. D. C. Bradley, R. H. Fricld, A. B. Holmes, J. Phys.: Condens. Matter 6, 1379 (1994); A. J. Heeger and G. Yu, U.S. Pat. No. 5,504,323 (April, 1996); R. H. Friend, A. B. Homes, D. D. C. Bradley, R. N. Marks, U.S. Pat. No. 5,523,555 (June, 1996).
Recent progress demonstrated that the photosensitivity in organic photodiodes can be enhanced under reverse bias; .about.90 mA/Watt was observed in ITO/MEH-PPV/Ca thin film devices at 10 V reverse bias (430 nm), corresponding to a quantum efficiency of &gt;20% el/ph [G. Yu, C. Zhang and A. J. Heeger, AppI. Phys. Lett. 64, 1540 (1994); A. J. Heeger and G. Yu, U.S. Pat. No. 5,504,323 (Apr. 2, 1996)]. In photodiodes fabricated with poly(3-octyl thiophene), photosensitivity greater than 0.3 A/Watt was observed over most of visible spectral range under -15 V bias with quantum efficiency over 80% cl/ph in the blue spectral region [G. Yu, H. Pakbaz and A. J. Heeger, Appl. Phys. Lett. 64, 3422 (1994)].
The photosensitivity in organic semiconductors at low bias fields can be enhanced by excited-state charge transfer; for example, by sensitizing the semiconducting polymer with acceptors such as C.sub.60 or its derivatives [N. S. Sariciftci and A. J. Heeger, U.S. Pat. 5,331,183 (Jul. 19, 1994); N. S. Sariciftci and A. J. Heeger, U.S. Pat. 5,454,880 (Oct. 3, 1995); N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Science 258, 1474 (1992); L. Smilowitz, N. S. Sariciftci, R. Wu, C. Gettinger, A. J. Heeger and F. Wudl, Phys. Rev. B 47, 13835 (1993); N. S. Sariciftci and A. J. Heeger, Intern. J. Mod. Phys. B 8, 237 (1994)]. Photoinduccd charge transfer prevents early time recombination and stabilizes the charge separation, thereby enhancing the carrier quantum yield for subsequent collection [B. Kraabcl, C. H. Lee, D. McBranch, D. Moses, N. S. Sariciftci and A. J. Heeger, Chem. Phys. Lett. 213, 389 (1993); N. S. Sariciftci, D. Braun, C. Zhlang and A. J. Heeger, Appl. Phys. Letters 62, 585 (1993); B. Kraabel, D. McBranch, N. S. Sariciftci, D. Moses and A. J. Heeger, Phys. Rev. B 50, 18543 (1994); C. H. Lee, G. Yu, D. Moses, K. Pakbaz, C. Zhang, N. S. Sariciftci, A. J. Heeger and F. Wudl, Phys. Rev. B. 48, 15425 (1993)]. By using charge transfer blends as the photosensitive material in photodiodes, external photosensitivitics of 0.2-0.3 A/W and external quantum yields of 50-80% el/ph have been achieved at 430 nm at only 2 V reverse bias [G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science 270, 1789 (1995); G. Yu and A. J. Heeger, J. Appl. Phys. 78, 4510 (1995). At the same wavelength, the photosensitivity of the UV-enhanced silicon photodiodes is .about.0.2 A/Watt, independent of bias voltage [S. M. Sze, Physics of Semiconductor Devices (Wiley, N.Y., 1981) Part 5]. Thus, the photosensitivity of thin film photodiodes made with polymer charge transfer blends is comparable to that of photodiodes made with inorganic semiconductor crystals. In addition to their high photosensitivity, these organic photodiodes show large dynamic range; relatively flat photosensitivity has been reported from 100 mW/cm.sup.2 down to nW/cm.sup.2 ; i.e., over eight orders of magnitude [G. Yu, H. Pakbaz and A. J. Heeger, Appl. Phys. Lett. 64, 3422 (1994); G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science 270, 1789 (1995); G. Yu and A. J. Heeger, J. AppI. Phys. 78, 4510 (1995)]. The dynamic range is, again, comparable to that of photodiodes made with inorganic semiconductors. Polymer photodetectors can be operated at room temperature, and the photosensitivity is relatively insensitive to the operating temperature [G. Yu, K. Pakbaz and A. J. Heeger, Appl. Phys. Lett. 64, 3422 (1994)].
As in the case for polymer light emitting devices [G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature 357, 477 (1992); A. J. Heeger and J. Long, Optics & Photonics News, Aug. 1996, p.24], high sensitivity polymer photodetectors can be fabricated in large areas by processing from solution at room temperature, they can be made in unusual shapes (e.g., on a hemisphere to couple with an optical component or an optical system), or they can be made in flexible or foldable forms. The processing advantages also enable one to fabricate the photosensors directly onto optical fibers. Similarly, polymer photodiodes can be hybridized with optical devices or electronic devices, such as integrated circuits on silicon wafers. These unique features make polymer photodiodes attractive for many novel applications.
In image sensing devices made with inorganic semiconductors, the photosensing layer must be pixelated to prevent the photoinduced charges from dispersing along the horizontal direction. Due to the relatively low carrier mobility in the organic semiconductors, the patterning of the photosensing layer becomes unnecessary in most image sensing applications, thereby simplifying the fabrication process significantly.